1. Field of the Invention
The present invention relates to range switching devices that perform switching of drive, reverse, and neutral ranges in an automatic transmission of a vehicle such as an automobile, and more particularly to a range switching device that can switch the ranges even when one or some valves fail.
2. Description of the Related Art
In an automatic transmission such as a CVT or a planetary gear step AT provided in an automobile or the like, the drive, reverse, and neutral ranges are switched by controlling the oil pressure supplied to engagement elements such as a forward clutch and a reverse clutch.
Switching of such drive, reverse, and neutral ranges has conventionally been performed by a manual valve connected by a mechanical linkage to an operation lever operated by the driver.
Recently, there has been proposed a technology so-called shift-by-wire system in which switching of running ranges is performed only by eclectic signals, without providing a mechanical linkage between the operation lever and the transmission.
As an example of a conventional technique relating to shift-by-wire systems of automatic transmission, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-128475 describes a range switching device in which spool valves are actuated by three solenoid valves to switch the oil pressure supplied to drive and reverse hydraulic servos.
Further, JP-A No. 2008-128473 describes a range switching device in which running ranges are switched by two solenoid valves and the running range can be maintained even when either one of the solenoid valves fails in the running range.
However, when one or some of solenoid valves fail, the technique described in JP-A No. 2008-128475 cannot switch the ranges, and thus the vehicle cannot run.
The technique described in JP-A No. 2008-128473 ensures minimal failsafe capability, thereby maintaining the running range during running and switching to the neutral range even when either one of the solenoid valves has failed. However, the neutral range is selected in this state, the range pressure cannot be supplied again, and thus the vehicle cannot run.